Steam turbine

ABSTRACT

A steam turbine  10  in an embodiment includes: an inner casing  21  surrounding a turbine rotor; an outer casing  20  composed of an upper half side outer casing  20   a  and a lower half side outer casing  20   b;  and an annular diffuser  50  through which steam passed through a turbine stage is discharged radially outward. A vertical distance H from a axis of a turbine rotor  23  to an inner wall of the upper half side outer casing  20   a,  an outermost diameter D of final stage rotor blades  22   a,  a blade height B of the final stage rotor blade  22   a,  and a distance W between inner walls in vertical and horizontal directions to the axis of the turbine rotor 23, at a bottom portion of the lower half side outer casing  20   b  forming a discharge port  60,  satisfy (H−D/2)/B≦1.7 and (W−D)/2B≧2.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-038725, filed on Feb. 24, 2012; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a steam turbine.

BACKGROUND

The thermal efficiency improvement of a steam turbine used in a thermalpower plant and the like has been an important task leading to effectiveuse of energy resources and reduction in carbon dioxide (CO₂) emission.

The thermal efficiency improvement of a steam turbine can be achieved byeffectively converting given energy to mechanical work. For this, it isnecessary to reduce various internal losses.

The internal losses in a steam turbine include a profile loss ascribableto the shape of blades, a steam secondary flow loss, a steam leakageloss, a turbine cascade loss based on a steam moisture loss and thelike, a passage part loss in passages typified by a steam valve and acrossover pipe, other than cascades, a turbine exhaust loss by a turbineexhaust hood, and so on.

Among these losses, the turbine exhaust loss is a large loss accountingfor 10 to 20% of the total internal losses. The turbine exhaust loss isa loss to occur between an outlet of a final stage and an inlet of acondenser. The turbine exhaust loss is further classified into a leavingloss, a hood loss, an annular area restriction loss, a turn-up loss, andso on. Among them, the hood loss is a pressure loss from an exhaust hoodto a condenser, and relies on the type, shape, and size of an exhausthood including a diffuser.

Generally, the pressure loss increases in proportion to the square of aflow velocity of steam. For reducing the pressure loss, it is effectiveto increase the size of an exhaust hood in an allowable range to reducethe flow velocity of steam. However, increasing the size of an exhausthood is subjected to the restrictions of a manufacturing cost, aninstallation space of a building, and so on. Increasing the size of anexhaust hood for reducing the hood loss is also subjected to suchrestrictions. Therefore, it becomes important to make an exhaust hood ina shape with a small pressure loss within a limited size.

For reducing the pressure loss in an exhaust hood, it is necessary to,in a diffuser, reduce the velocity of steam sufficiently to recover astatic pressure and reduce the pressure loss on the downstream side ofthe diffuser. For this, various examinations have been made in order toreduce the turbine exhaust loss.

FIG. 13 is a view showing part of a meridian cross section in thevertical direction in an upper half portion of a conventional steamturbine 200. As shown in FIG. 13, steam having passed through a rotorblade 210 forming a final turbine stage is guided to an annular diffuser213 formed of a steam guide 211 and a bearing cone 212.

The steam to be discharged from the upper half side of the diffuser 213flows radially outward in a radial and substantially equal manner asindicated by the arrow in FIG. 13 without directly flowing to an outletof an exhaust hood. The steam having flowed out from the diffuser 213flows between an outer casing 214 and an inner casing 215. Particularly,the steam having flowed vertically upward collides with an upper halfside flow guide 216 provided on the outer casing 214 of the upper halfside and an outer surface of the inner casing 215, and thereby the flowdirection is turned downward. Then, the steam having had its flow turneddownward is guided to a condenser (not shown) installed below a turbinerotor 217, namely under the steam turbine 200.

As described above, in the steam turbine 200 in which the exhaust hoodguiding the steam to be discharged to the condenser installed below isprovided, of the steam having flowed radially outward, the steam havingflowed vertically upward in particular collides with the upper half sideflow guide 216 and the inner casing 215, and thereby the flow directionis turned downward. Therefore, the steam discharged from the upper halfside of the diffuser 213 undergoes a large pressure loss rather thansteam discharged from the lower half side of the diffuser 213. Thereby,the pressure loss of the flow of the steam in the exhaust hoodincreases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a meridian cross section in the verticaldirection, of a steam turbine in a first embodiment, including a centeraxis of a turbine rotor.

FIG. 2 is a perspective view showing part of an exhaust hood in thesteam turbine in the first embodiment.

FIG. 3 is a view showing part of a meridian cross section in thevertical direction, of the exhaust hood in the steam turbine in thefirst embodiment, including the center axis of the turbine rotor.

FIG. 4 is a view showing part of a meridian cross section in thehorizontal direction, of the exhaust hood in the steam turbine in thefirst embodiment, including the center axis of the turbine rotor.

FIG. 5 is a view showing a cross section, taken along A-A in FIG. 3,showing the steam turbine in the first embodiment.

FIG. 6 is a view showing part of a meridian cross section in thevertical direction, of an exhaust hood in a steam turbine in a secondembodiment, including a center axis of a turbine rotor.

FIG. 7 is a view showing part of a meridian cross section in thehorizontal direction, of the exhaust hood in the steam turbine in thesecond embodiment, including the center axis of the turbine rotor.

FIG. 8 is a view showing a cross section, taken along B-B in FIG. 6,showing the steam turbine in the second embodiment.

FIG. 9 is a view showing the relationship between (H−D/2)/B and aturbine exhaust loss.

FIG. 10 is a view showing the relationship between (W−D)/2B and theturbine exhaust loss.

FIG. 11 is a view showing the relationship between (H−C/2)/B and theturbine exhaust loss.

FIG. 12 is a view showing the relationship between (W−C)/2B and theturbine exhaust loss.

FIG. 13 is a view showing part of a meridian cross section in thevertical direction in an upper half portion of a conventional steamturbine.

DETAILED DESCRIPTION

In one embodiment, a steam turbine having a path to guide steam havingpassed through a final turbine stage to a condenser provided below. Thissteam turbine includes: an inner casing surrounding a turbine rotor andcomposed of an upper half side inner casing and a lower half side innercasing; and an outer casing surrounding the inner casing and composed ofan upper half side outer casing and a lower half side outer casing.Further, the steam turbine includes an annular diffuser that is providedon a downstream side of the final turbine stage and through which thesteam having passed through the final turbine stage is dischargedradially outward.

Then, a vertical distance H from a center axis of the turbine rotor toan inner wall of the upper half side outer casing, an outermost diameterD of final stage rotor blades forming the final turbine stage, a bladeheight B of the final stage rotor blade, and a distance W between innerwalls in vertical and horizontal directions to the center axis of theturbine rotor, at a bottom portion of the lower half side outer casingforming a discharge port through which the steam is discharged to thecondenser, are set so as to satisfy (H−D/2)/B≦1.7 and (W−D)/2B≧2.

Hereinafter, there will be explained embodiments of the presentinvention with reference to the drawings.

First Embodiment

FIG. 1 is a view showing a meridian cross section in the verticaldirection, of a steam turbine 10 in the first embodiment, including acenter axis 0 of a turbine rotor 23. FIG. 2 is a perspective viewshowing part of an exhaust hood 70 in the steam turbine 10 in the firstembodiment. Note that in FIG. 2, a state which an outer casing 20 isremoved from the steam turbine 10 is shown.

FIG. 3 is a view showing part of a meridian cross section in thevertical direction, of the exhaust hood 70 in the steam turbine 10 inthe first embodiment, including the center axis O of the turbine rotor23. FIG. 4 is a view showing part of a meridian cross section in thehorizontal direction, of the exhaust hood 70 in the steam turbine 10 inthe first embodiment, including the center axis 0 of the turbine rotor23. FIG. 5 is a view showing a cross section, taken along A-A in FIG. 3,showing the steam turbine 10 in the first embodiment. Note that FIG. 4is a cross section of the upper half side of the exhaust hood 70 seenfrom the lower half side. FIG. 5 is shown in a manner that part of theconstitution is omitted as a matter of convenience.

Here, as the steam turbine 10, a double flow exhaust type low-pressureturbine having a downward exhaust type exhaust hood is explained as anexample.

As shown in FIG. 1, in the outer casing 20, an inner casing 21 isprovided, and the steam turbine 10 is provided with what is called adouble casing structure. The outer casing 20 and the inner casing 21 areeach formed in a structure divided into upper and lower two parts by ahorizontal plane including the center axis 0 of the turbine rotor 23.The outer casing 20 is composed of an upper half side outer casing 20 aand a lower half side outer casing 20 b. The inner casing 21 is composedof an upper half side inner casing 21 a and a lower half side innercasing 21 b.

In the inner casing 21, the turbine rotor 23 having rotor blades 22implanted therein is penetratingly installed. The plural rotor blades 22are implanted in the circumferential direction to thereby form a rotorblade cascade. A plurality of stages of the rotor blade cascades isprovided in the axial direction of the turbine rotor 23. The turbinerotor 23 is rotatably supported by rotor bearings 24. Incidentally,among the rotor blades 22, the rotor blade provided on a final turbinestage is set as a final stage rotor blade 22 a here.

On an inner circumference of the inner casing 21, nozzles (stationaryblades) 26 supported by diaphragms 25 a and 25 b are provided so as tobe alternate with the rotor blades 22 in the axis direction of theturbine rotor 23. The plural nozzles 26 are implanted in thecircumferential direction to thereby form a nozzle cascade (stationaryblade cascade). The nozzle cascade and the rotor blade cascadepositioned on the immediately downstream side of the nozzle cascade formone turbine stage. Note that the inner casing 21 is supported by theouter casing 20, for example.

In the center of the steam turbine 10, an intake chamber 28 into whichsteam through a crossover pipe 27 is introduced is provided. The steamis introduced into the right and left turbine stages from this intakechamber 28 in a distributed manner.

As shown in FIG. 1 to FIG. 4, on the downstream side of the finalturbine stage, an annular diffuser 50 through which the steam isdischarged radially outward is formed by a steam guide 30 of the outercircumference side and a bearing cone 40 of the inner circumferenceside. Note that inside the bearing cone 40, the rotor bearing 24, andthe like are provided.

As shown in FIG. 3 and FIG. 4, the steam guide 30 and the bearing cone40 are each formed in a structure divided into upper and lower two partsby the horizontal plane including the center axis 0 of the turbine rotor23. For example, the cylindrical steam guide 30 in a bell mouthed andexpanded open shape is formed by the steam guide 30 of the upper halfside and the steam guide 30 of the lower half side. Similarly, thecylindrical bearing cone 40 in a bell mouthed and expanded open shape isformed by the bearing cone 40 of the upper half side and the bearingcone 40 of the lower half side. Then, the annular diffuser 50 is formedby the cylindrical steam guide 30 and the cylindrical bearing cone 40provided inside thereof. Note that of the steam guide 30, the formationof the upper half side and the formation of the lower half side are thesame, and of the bearing cone 40, the formation of the upper half sideand the formation of the lower half side are the same.

As shown in FIG. 3 and FIG. 4, the annular diffuser 50 is preferablyformed so that at an outlet of the annular diffuser 50, the flow of thesteam may flow radially outward, and for example, an endmost portion 30a of the outlet side of the steam guide 30 may become the same in thehorizontal position as an endmost portion 40 a of the outlet side of thebearing cone 40. That is, a radial distance from the center axis O ofthe turbine rotor 23 to the endmost portion 30 a and a radial distancefrom the center axis O of the turbine rotor 23 to the endmost portion 40a are preferably formed equally. The steam having performed expansionwork in the turbine stages passes through the previously describedannular diffuser 50 to be discharged radially outward from the outlet ofthe annular diffuser 50.

As shown in FIG. 2, in a bottom portion of the outer casing 20, adischarge port 60 through which the steam is discharged to a condenser(not shown) provided under the steam turbine 10 is formed. Here, a steampassage where the steam having passed through the final turbine stageflows to be discharged from the discharge port 60 of the outer casing 20functions as the exhaust hood 70.

The exhaust hood 70 includes a large space portion surrounded by theupper half side outer casing 20 a having an upper half side keel rib 80a and the lower half side outer casing 20 b having a lower half sidekeel rib 80 b as shown in FIG. 2 and FIG. 5, for example. Further, onthe upper half side outer casing 20 a, there is provided an upper halfside flow guide 81 turning the steam having flowed vertically upwardfrom the annular diffuser 50 of the upper half side downward.

On the bottom portion side of the lower half side outer casing 20 b,besides the above-described lower half side keel rib 80 b, partitionparts 82 and 83, and so on are provided. As above, the upper half sidekeel rib 80 a, the lower half side keel rib 80 b, the partition parts 82and 83, and so on are provided, thereby making the exhaust hood 70 in astructure durable against pressing pressure such as outside airpressure.

Here, as shown in FIG. 3 and FIG. 5, a vertical distance from the centeraxis O of the turbine rotor 23 to an inner wall of the upper half sideouter casing 20 a is set to H. As shown in FIG. 3 and FIG. 4, anoutermost diameter of the final stage rotor blades 22 a forming thefinal turbine stage is set to D. Here, the outermost diameter D is equalto a diameter of a circle made by blade edges of the final stage rotorblades 22 a when the final stage rotor blades 22 a rotate. Further, ablade height of the final stage rotor blade 22 a is set to B. Here, theblade height B is a distance from a blade root to a blade edge as shownin FIG. 3 and FIG. 4, and functions as a representative dimensionindicating an inlet of the exhaust hood 70 (an inlet of the annulardiffuser 50).

The vertical distance H, the outermost diameter D of the final stagerotor blades 22 a, and the blade height B of the final stage rotor blade22 a described above are set so as to satisfy the following expression(1).

(H−D/2)/B≦1.7   Expression (1)

Further, as shown in FIG. 5, in the bottom portion of the lower halfside outer casing 20 b, the discharge port 60 through which the steam isdischarged to the condenser (not shown) is formed. Then, a distancebetween inner walls 90 a and 90 b in the vertical and horizontaldirections to the center axis O of the turbine rotor 23, at the bottomportion (the discharge port 60) of the lower half side outer casing 20 bis set to W. This distance W, the outermost diameter D of the finalstage rotor blades 22 a and the blade height B of the final stage rotorblade 22 a that are described previously are set so as to satisfy thefollowing expression (2).

(W−D)/2B≧2   Expression (2)

That is, the steam turbine 10 is constituted so as to satisfy Expression(1) and Expression (2) described above.

Here, Expression (1) described above is satisfied, thereby narrowing avertical upper space between the upper half side outer casing 20 a andthe upper half side inner casing 21 a, which is formed verticallyupward. Then, a flow rate of steam ST1 flowing into this space isreduced (see FIG. 5). That is, a flow rate of the steam passing througha gap between the upper half side outer casing 20 a and the endmostportion 30 a of the downstream side of the steam guide 30 and collidingwith the upper half side flow guide 81 and the upper half side innercasing 21 a to thereby have its flow turned downward is reduced (seeFIG. 3). Therefore, it is possible to reduce, in the exhaust hood 70, apressure loss of the steam having flowed out from the annular diffuser50 of the upper half side.

Here, the lower limit value of (H−D/2)/B is 0.5 or so due to making theexhaust hood 70 in a structure durable against pressing pressure such asoutside air pressure by securing the height of the upper half side keelrib 80 a.

On the other hand, Expression (2) described above is satisfied, therebyenlarging a horizontal direction space between the upper half side outercasing 20 a and the upper half side inner casing 21 a, which is formedin the horizontal direction. Then, a flow rate of steam ST2 flowing intothis space is increased (see FIG. 5). That is, since the flow rate ofthe steam ST1 is reduced, the flow rate of the steam ST2 flowing intothe horizontal direction space is increased. The steam ST2 having flowedinto the horizontal direction space is guided to the discharge port 60of the lower half side outer casing 20 b without colliding with theupper half side flow guide 81 and the upper half side inner casing 21 a.Therefore, it is possible to reduce, in the exhaust hood 70, thepressure loss of the steam having flowed out from the annular diffuser50 of the upper half side.

Here, the upper limit value of (W−D)/2B is 4 or so for the reason ofpreventing occurrence of an enlargement loss caused by suddenenlargement of a channel area with respect to the flow of the steamhaving flowed out from the annular diffuser 50.

Incidentally, the steams ST1 and ST2 having flowed out from the annulardiffuser 50 of the upper half side flow together with steam ST3 havingflowed out from the annular diffuser 50 of the lower half side to bedischarged from the discharge port 60 to the condenser (not shown).

As described above, according to the steam turbine 10 in the firstembodiment, as for the steam flowing in the radial direction from theannular diffuser 50 of the upper half side in a radial manner, it ispossible to reduce the flow rate of the steam ST1 flowing into thenarrow vertical upper space and to increase the flow rate of the steamST2 flowing into the large horizontal direction space. This makes itpossible to reduce, in the exhaust hood 70, the pressure loss of thesteam having flowed out from the annular diffuser 50 of the upper halfside.

Second Embodiment

FIG. 6 is a view showing part of a meridian cross section in thevertical direction, of an exhaust hood 70 in a steam turbine 11 in thesecond embodiment, including a center axis O of a turbine rotor 23. FIG.7 is a view showing part of a meridian cross section in the horizontaldirection, of the exhaust hood 70 in the steam turbine 11 in the secondembodiment, including the center axis O of the turbine rotor 23. FIG. 8is a view showing a cross section, taken along B-B in FIG. 6, showingthe steam turbine 11 in the second embodiment. Note that FIG. 7 is across section of the upper half side of the exhaust hood 70 seen fromthe lower half side. FIG. 8 is shown in a manner that part of theconstitution is omitted as a matter of convenience. Further, the sameconstituent parts as those of the steam turbine 10 in the firstembodiment are denoted by the same reference signs, and a duplicatedescription will be omitted or simplified.

In the steam turbine 11 in the second embodiment, as shown in FIG. 6 andFIG. 8, a vertical distance from the center axis O of the turbine rotor23 to an inner wall of an upper half side outer casing 20 a is set to H.As shown in FIG. 6 to FIG. 8, an outermost diameter at an endmostportion 30 a of the outlet side (downstream side) of a steam guide 30forming an outlet of an annular diffuser 50 is set to C. Further, ablade height of a final stage rotor blade 22 a is set to B. Here, theblade height B is a distance from a blade root to a blade edge as shownin FIG. 6 and FIG. 7.

The vertical distance H, the outermost diameter C of the endmost portion30 a, and the blade height B of the final stage rotor blade 22 adescribed above are set so as to satisfy the following expression (3).

0.2≦(H−C/2)/B≦0.85   Expression (3)

Further, as shown in FIG. 8, in a bottom portion of a lower half sideouter casing 20 b, a discharge port 60 through which steam is dischargedto a condenser (not shown) is formed. Then, a distance between innerwalls 90 a and 90 b in the vertical and horizontal directions to thecenter axis 0 of the turbine rotor 23, at the bottom portion (thedischarge port 60) of the lower half side outer casing 20 b is set to W.This distance W, the outermost diameter C of the endmost portion 30 aand the blade height B of the final stage rotor blade 22 a that aredescribed previously are set so as to satisfy the following expression(4).

1.1≦(W−C)/2B≦1.8   Expression (4)

That is, the steam turbine 11 is constituted so as to satisfy Expression(3) and Expression (4) described above.

Here, as shown in FIG. 6 and FIG. 7, the flow of steam having flowedradially outward from the outlet of the annular diffuser 50 of the upperhalf side has its flow direction turned at 90 degrees, and the steamflows into a gap between the upper half side outer casing 20 a and theendmost portion 30 a of the steam guide 30. In the case of this gapbeing narrow, the flow of the steam having flowed radially outward in aradial manner from the outlet of the annular diffuser 50 of the upperhalf side does not flow through this gap easily. Thereby, a largepressure loss occurs.

On the other hand, in the case of the gap between the upper half sideouter casing 20 a and the endmost portion 30 a of the steam guide 30being large, the distance between the upper half side outer casing 20 aand the endmost portion 30 a of the steam guide 30 is lengthened.Therefore, the steam having flowed out from the outlet of the annulardiffuser 50 of the upper half side diffuses suddenly in a space.Thereby, a pressure loss caused by sudden enlargement of the flowoccurs.

Further, the distance between the outlet of the annular diffuser 50 ofthe upper half side and an inner wall surface of the upper half sideouter casing 20 a facing this outlet is long, and thus the inner wallsurface of the upper half side outer casing 20 a does not function as aflow guide guiding the flow of the steam in a predetermined direction.Therefore, it makes it difficult to turn the flow of the steam havingflowed radially outward from the outlet of the annular diffuser 50 ofthe upper half side at 90 degrees to appropriately let the steam flowinto the gap between the upper half side outer casing 20 a and theendmost portion 30 a of the steam guide 30. Thereby, the flow of thesteam is inclined toward the bearing cone 40 side. Therefore, it becomesimpossible to effectively use an exhaust area of the discharge port 60,resulting in that the pressure loss increases.

As above, with regard to the gap between the endmost portion 30 a of thesteam guide 30 and the upper half side outer casing 20 a facing this, anoptimal value exists. That is, even when the gap between the endmostportion 30 a of the steam guide 30 and the upper half side outer casing20 a facing this is smaller or larger than this optimal value, thepressure loss increases.

Further, this gap is narrowed as much as possible, thereby making itpossible to reduce the flow rate of the steam ST1 colliding with anupper half side flow guide 81 and an upper half side inner casing 21 ato thereby have its flow turned downward (see FIG. 8). This makes itpossible to reduce, in the exhaust hood 70, the pressure loss of thesteam having flowed out from the annular diffuser 50 of the upper halfside. Therefore, the gap between the endmost portion 30 a of the steamguide 30 and the upper half side outer casing 20 a facing this isreduced as much as possible to such an extent that the flow of the steamis not hindered, and the range of (H−C/2)/B is set.

Further, a horizontal direction space between the upper half side outercasing 20 a and the upper half side inner casing 21 a, which is formedin the horizontal direction, is enlarged to such an extent that a suddenenlargement loss of the flow does not occur, and thereby the flow rateof the steam ST2 flowing into this space is increased (see FIG. 8). Thatis, since the flow rate of the steam ST1 is reduced, the flow rate ofthe steam ST2 flowing into the horizontal direction space is increased.

The steam ST2 having flowed into the horizontal direction space isguided to the discharge port 60 of the outer casing 20 without collidingwith the upper half side flow guide 81 and the upper half side innercasing 21 a. Therefore, it is possible to reduce, in the exhaust hood70, the pressure loss of the steam having flowed out from the annulardiffuser 50 of the upper half side. Based on the description above, therelational expressions of Expression (3) and Expression (4) describedabove can be obtained.

As described above, according to the steam turbine 11 in the secondembodiment, it is possible to reduce the gap between the endmost portion30 a of the steam guide 30 and the upper half side outer casing 20 afacing this as much as possible to such an extent that the flow of thesteam is not hindered. Therefore, it is possible to reduce the flow rateof the steam ST1 flowing into the narrow vertical upper space and toincrease the flow rate of the steam ST2 flowing into the largehorizontal direction space. This makes it possible to reduce, in theexhaust hood 70, the pressure loss of the steam having flowed out fromthe annular diffuser 50 of the upper half side.

(Evaluation of a Turbine Exhaust Loss) (1) (H−D/2)/B, (W−D)/2B and aTurbine Exhaust Loss

FIG. 9 is a view showing the relationship between (H−D/2)/B and theturbine exhaust loss. FIG. 10 is a view showing the relationship between(W−D)/2B and the turbine exhaust loss. These relationships are resultsobtained from a model test by a scale model and a numerical analysis.

Incidentally, in the evaluations according to FIG. 9 and FIG. 10, theturbine exhaust loss was evaluated by changing both values of (H−D/2)/Band (W−D)/2B. Concretely, with reducing the value of (H−D/2)/B, thevalue of (W−D)/2B was increased. For example, when (H−D/2)/B was 1.7,(W−D)/2B was 2, and with reducing (H−D/2)/B from 1.7, (W−D)/2B wasincreased from 2.

As shown in FIG. 9, when the value of (H−D/2)/B exceeds 1.7, the turbineexhaust loss increases rapidly. Further, as shown in FIG. 10, when(W−D)/2B is less than 2, the turbine exhaust loss increases rapidly.Therefore, it is found that (H D/2)/B is set to 1.7 or less and (W−D)/2Bis set to 2 or more, thereby making it possible to reduce the turbineexhaust loss.

(2) (H−C/2)/B, (W−C)/2B and the Turbine Exhaust Loss

FIG. 11 is a view showing the relationship between (H−C/2)/B and theturbine exhaust loss. FIG. 12 is a view showing the relationship between(W−C)/2B and the turbine exhaust loss. These relationships are resultsobtained in an actual machine.

Incidentally, in the evaluations according to FIG. 11 and FIG. 12, theturbine exhaust loss was evaluated by changing both values of (H−C/2)/Band (W−C)/2B. Concretely, with increasing the value of (H−C/2)/B, thevalue of (W−C)/2B was reduced. For example, when (H−C/2)/B was 0.2,(W−C)/2B was 1.8, and with increasing (H−C/2)/B from 0.2, (W−C)/2B wasreduced from 1.8. (W−C)/2B when (H−C/2)/B is 0.85 is 1.1.

As shown in FIG. 11, when (H−C/2)/B is 0.6 or so, a maximum staticpressure recovery amount is obtained, and when (H−C/2)/B deviates fromthe value, the static pressure recovery amount reduces and the turbineexhaust loss increases. Here, according to a normal turbine designstandard, a design allowing up to the static pressure recovery amountreduced by 20% from the maximum static pressure recovery amount is made.In FIG. 11, the static pressure recovery amount reduced by 20% from themaximum static pressure recovery amount is indicated by a dotted line.Therefore, it is found that (H−C/2)/B is set to not less than 0.2 normore than 0.85 and (W−C)/2B is set to not less than 1.1 nor more than1.8, thereby making it possible to obtain the static pressure recoveryamount equal to or more than the previously described turbine designstandard value and reduce the turbine exhaust loss.

According to the explained embodiments above, it becomes possible tosuppress the pressure loss of the flow of the steam in the exhaust hoodand reduce the turbine exhaust loss.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A steam turbine having a path to guide steamhaving passed through a final turbine stage to a condenser providedbelow, the steam turbine, comprising: an inner casing surrounding aturbine rotor and composed of an upper half side inner casing and alower half side inner casing; an outer casing surrounding the innercasing and composed of an upper half side outer casing and a lower halfside outer casing; and an annular diffuser that is provided on adownstream side of the final turbine stage and through which the steamhaving passed through the final turbine stage is discharged radiallyoutward, wherein a vertical distance H from a center axis of the turbinerotor to an inner wall of the upper half side outer casing, an outermostdiameter D of final stage rotor blades forming the final turbine stage,a blade height B of the final stage rotor blade, and a distance Wbetween inner walls in vertical and horizontal directions to the centeraxis of the turbine rotor, at a bottom portion of the lower half sideouter casing forming a discharge port through which the steam isdischarged to the condenser, satisfy the following expression (1) andexpression (2).(H−D/2)/B≦1.7   Expression (1)(W−D)/2B≧2   Expression (2)
 2. A steam turbine having a path to guidesteam having passed through a final turbine stage to a condenserprovided below, the steam turbine, comprising: an inner casingsurrounding a turbine rotor and composed of an upper half side innercasing and a lower half side inner casing; an outer casing surroundingthe inner casing and composed of an upper half side outer casing and alower half side outer casing; and an annular diffuser that is providedon a downstream side of the final turbine stage and is formed by a steamguide and a bearing cone inside the steam guide and through which thesteam having passed through the final turbine stage is dischargedradially outward, wherein a vertical distance H from a center axis ofthe turbine rotor to an inner wall of the upper half side outer casing,an outermost diameter C at an endmost portion of the steam guide formingan outlet of the annular diffuser, a blade height B of a final stagerotor blade forming the final turbine stage, and a distance W betweeninner walls in vertical and horizontal directions to the center axis ofthe turbine rotor, at a bottom portion of the lower half side outercasing forming a discharge port through which the steam is discharged tothe condenser, satisfy the following expression (3) and expression (4).0.2≦(H−C/2)/B≦0.85   Expression (3)1.1≦(W−C)/2B≦1.8   Expression (4)